Controlling NOx emissions in diesel engines has posed significant challenges to the automotive industry. While emission control devices, such as NOx catalysts, can be used, these devices may be insufficient to meet ever-increasing emission standards.
A method to reduce NOx in diesel engines is the use of exhaust gas recirculation (EGR). EGR reduces NOx emissions during steady, or near steady, engine operation.
However, under transient engine operation in which a vehicle is required to accelerate, EGR can limit the performance of the vehicle by reducing the amount of airflow through the engine. EGR reduces airflow by displacing air in the combustion chamber, heating up the intake charge, and redirecting exhaust gas that would normally go through the turbocharger to the intake manifold. This last effect reduces the energy flow through the turbine, thus restricting the engine's ability to create boost. This phenomenon can be dubbed “the EGR-Boost” tradeoff.
Typically, conventional diesel systems suspend the use of EGR in order to accelerate aggressively. However, the inventors herein have recognized that without EGR, NOx emissions (concentration) increase dramatically. This comes at a time when the air mass flow rates are very high, causing NOx production to spike. Thus, the place where EGR is most needed is the place where it is not used.
The difficulty of this problem can be further appreciated by considering two types of EGR system that could be used with turbo-charged engines: the high pressure system and the low pressure system. The inventors herein have recognized the following disadvantages with each system.
In heavy duty applications, high pressure EGR systems have difficulty providing sufficient EGR flow at high load conditions, especially at low speed, since there is a negative pressure differential between the exhaust and intake manifold. One method to improve EGR flow is to throttle the engine. However, throttling the engine increases engine pumping work and decreases the gas flow to the engine.
Similarly, low pressure EGR systems allow only limited amounts of EGR under light load conditions, especially at low speed, since there is little pressure differential. Further, the low pressure EGR system adds significant purging volume that causes delays when trying to accelerate.